This invention relates to digital printing generally, and is more specifically directed to a method of printing an image onto a substrate by digital means using liquid reactive ink, and subsequently activating the ink to permanently fix the printed image.
Words, images and designs are frequently printed onto clothing and other textile materials, as well as other objects. The silk screen process is a stencil process well known in the art for printing images directly onto textiles as well as indirectly via transfer paper. The two main types of screen printing inks are pigmented emulsions and plastisol inks. The emulsion inks are typically based on aqueous dispersions of a binder and cross-linking agent. The emulsion inks are used for direct printing onto all types of fabric. After printing, the prints are fixed onto the textile by heat. The plastisol inks are typically vinyl resin dispersed in plasticizer. They may be applied directly onto the fabric or used as a transfer. When used as a transfer, the ink is screen printed onto a release paper, cured to a dry film, and stored until transferred to fabric by a heat transfer process.
Traubel et al., U.S. Pat. No. 5,556,935 discloses a screen printing paste containing polyisocyanate mixtures comprising hydrophilic polyisocyanates, hydrophilic polyisocyanates containing carbidiimide groups, and/or polyepoxide compounds as cross-linking agents. However, these mixtures are insufficient under practical conditions; in particular, the finished printing pastes are not stable on storage. A continuous loss of isocyanate groups takes place through reaction of free isocyanate groups with water, which ultimately leads to products that are inactive with respect to crosslinking.
Uhl et al., U.S. Pat. No. 4,849,262 discloses a screen printing paste and aqueous dyeing liquor containing particle dispersions of polyisocyanate cross-linking agent in a deactivated (partially blocked) form. The deactivation of the particle surfaces is achieved by the dispersion of polyisocyanates in the presence of media that are reactive with isocyanate. Only the isocyanate groups present on the surface of the particles react with the deactivating agent. The rest of the polyisocyanate molecules in the interior of the particle remain unreacted. The deactivation compounds form a sort of polymer shell on the surface of the polyisocyanate particles, which is removed with heat above 60xc2x0 C. Apparently this shell imparts a prolonged pot life to the printing pastes, compared to prior art.
Reiff et al., U.S. Pat. No. 5,607,482 discloses a screen printing paste containing a hydrophilic polyisocyanate prepolymer as a cross-linking agent. The isocyanate groups of the prepolymer are chemically blocked to prevent reaction. The blocking agent is removed with heat. Such print pastes show prolonged pot life due to both the complete blocking of reactive groups and the reduced number of reactive groups in the larger molecular weight prepolymer. Such a paste design is limits to silk-screen and/or offset printing only due to physical properties of the paste which include high solid percentage, high viscosity, and relatively large component particle sizes. The patent does not teach how to design an ink system which satisfies the conditions suitable for a digital printing environment. Both physical and chemical properties need to be carefully adjusted in order to meet these conditions. Furthermore, Reiff teaches only aqueous based paste system where water is an essential part of the system.
Although the silk-screen process is the predominant printing method for textiles, it does have certain disadvantages in today""s digital computer age. Silk-screen is an analog printing process. As such it is not capable of matching the quality of digital graphics, especially photographic images. Nor can the process quickly or easily make changes to the print design. The use of digital computer technology allows a virtually instantaneous printing of images, each of which may be different from the other. For example, video cameras or scanning may be used to capture an image to a computer. Graphic art software allows nearly unlimited manipulation of the image. The image is typically printed by a computer driven printer that will print process color inks from multiple ink reservoirs. Printing may be done directly onto the final substrate, such as a textile fabric, or onto an intermediate substrate, such as paper, followed by a transfer process.
The digital printing field utilizes various combinations of reactive species in an effort to create an ink with increased bonding to a paper substrate. Hackleman et al., U.S. Pat. No. 4,694,302 discloses an ink jet printing system which contains a reactive species present in the ink itself (one component system) or in a separate reservoir (two component system). The reaction of the reactive species with the substrate (one component system) or with the ink (two component system) forms a polymer which binds the dye onto the surface of the substrate. Titterington et al., U.S. Pat. Nos. 5,380,769 and 5,645,888 describe ink jet ink compositions which contain at least two reactive components, a base ink which contains a crosslinkable constituent, and a curing component which is a crosslinking agent. The base ink and curing component are applied to a receiving substrate separately. The base ink is preferably applied first. Upon exposure of the base ink to the curing component, a durable, cross-linked ink is produced. Under circumstances where the crosslinkable constituent and the crosslinking agent are unreactive until a catalyst or other curing agent is introduced, both the crosslinkable constituent and a crosslinking agent may be incorporated in the ink carrier while the catalyst serves as a curing component.
Titterington et al., U.S. Pat. No. 5,645,888 applies the base ink component to an intermediate support surface to be subsequently transferred to a desired receiving surface. The curing component may be applied to the intermediate support surface directly or in combination with an ink release agent.
In Hackleman et al., U.S. Pat. No. 4,694,302, Titterington et al., U.S. Pat. No. 5,380,769 and Titterington et al., U.S. Pat. No. 5,645,888, the printing process brings into contact all reactive components, which initiates the crosslinking reaction. For images printed onto an intermediate support, transfer to the desired receiving surface must be performed while the crosslinking reaction is still occurring, to achieve the maximum adherence and durability of the ink to the surface. There is no ability for long-term storage of printed images in an unreacted form.
Held, U.S. Pat. No. 5,853,861 discloses a digital printing process, specifically ink jet printing, for direct printing onto a textile, rather than paper. The ink contains an aqueous carrier, a pigment and a polymer having acid, base, epoxy or hydroxyl functional moieties. The textile is pre-treated with a solution of either an organometallic crosslinking agent or an isocyanate crosslinking agent. Upon exposure of the printed image to an external energy source, the crosslinking agent will react with the textile and the polymer in the ink to fix the image. However, the area of the textile outside the printed image will also react with the crosslinking agent, possibly creating discoloration and a harsh hand. Also, as in Traubel et al., U.S. Pat. No. 5,556,935, the crosslinking agents are not blocked from reaction. Therefore the life of the crosslinking agent in solution and on the fabric is severely reduced. Continuous loss of reactive groups ultimately leads to a textile which is inactive with respect to crosslinking.
Various inks for use in ink jet printers are known. Solvent-based inks, including both aqueous and non-aqueous inks, are well known. Solvent-based inks can be printed using piezoelectrically actuated printheads. Images are formed by the ejection of ink droplets onto a receiving surface and subsequent removal, such as by evaporation or diffusion, of one to all of the solvents. Phase change inks are solid at ambient temperatures and liquid at the elevated operating temperatures of an ink jet printing device. Ink jet droplets in the liquid phase are ejected from the printing device at an elevated operating temperature and rapidly solidify when they contact with the surface of a substrate to form the predetermined pattern. Thermal or bubble-jet devices use heating element inside of the printing device to create instantaneous vapor bubbles and propel the ink to form small droplets from the print head and form digitally oriented image. Continuous inkjet devices use printing ink with charging characteristics and with a continuous ink droplet flow through the printing transducer. By controlling the polarity of an electrode prior to the emit nozzle it determines whether a particular ink droplet ejects or not therefore an image is formed. The present invention is suitable for one to all the above printing technologies.
Another objective of the present invention is to present a printing process which uses a reactive ink system which is a liquid at the temperature at which the printing device operates. The ink system is an emulsion or emulsion-like system, so that the ink system is stabile during storage and printing according to the processes described herein. Characteristics to be considered in preparing the ink system include, but are not limited to, particle size and particle size distribution, pH value, optical density, charge density, viscosity, and surface energy.